1. Field of the Invention
This invention relates to raster scan lithography as used for producing masks and wafers for integrated circuit fabrication, and more specifically to raster scan electron beam systems.
2. Description of the Prior Art
It is well known to use particle (electron or ion) or light beam exposure systems for the manufacture of microminiature electronic devices (integrated circuits). Often these systems use raster scan beam lithography methods. Such lithographic employing a controllable electron beam, sometimes called E-beam machines, for fabrication of integrated circuits are well known; one such system is described in U.S. Pat. No. 3,900,737 to Collier et al. and an other in U.S. Pat. No. 3,801,792 to Lin.
In these machines a medium of resist (or photosensitive) material upon which the electron beam is to perform its writing operation is provided overlying a substrate which is to become a mask for imaging a wafer or which is the wafer itself (direct writing). The medium with its underlying substrate is positioned on a motor-driven stage which is moved continuously in synchronism as the beam is scanned in a raster fashion (a raster scan) and in a direction perpendicular to the stage motion. In practice, the diameter of the round electron beam spot, also called a "Gaussian spot" or "pixel", focussed on the resist layer, is of the order of (but not necessarily equal to) the writing address dimension (or address unit) of the cartesian grid on which it is written. Adjacent rows of pixels in the stage travel direction define the width of a "feature" and a length of the feature is formed by a number of pixels in the raster scan direction. The feature is for instance an element of the integrated circuit such as a conductive interconnect or a portion of a transistor. In practice, adjacent "on" pixels in the same raster scan are not separately scanned; instead the beam is kept on until an "off" pixel is encountered. For the purposes of this disclosure, descriptions are given in terms of the normal full pixelization representation. The turning on and off of the beam is achieved by a beam blanker which is a well known portion of an E-beam machine, one example of which is shown in U.S. Pat. No. 5,276,330, patent application Ser. No. 07/706,612, filed May 29, 1991, entitled "High Accuracy Beam Blanker" incorporated by reference herein, invented by Mark A. Gesley. The pattern on the resist defined by the beam scan and by the stage movement is determined by the system control equipment which includes certain computer software programs.
Several modifications of this system are used to achieve feature edge position increment which is finer than the raster grid, i.e. the edges of a feature fall between the center points of the cartesian grid defined by the pixel centerpoints. One example of such a system is described in Berglund et al., U.S. Pat. No. 5,103,101 incorporated by reference and defining a system for multiphase printing for E-beam lithography. Another such system is described in U.S. Pat. No. 4,879,605 to Warkentin et al. for a rasterizing system using an overlay of bit map low address resolution databases, using a multi-pass laser beam writing technique to vary the dosage at a feature edge dependent upon the desired location of a given edge.
Another relevant disclosure is U.S. Pat. No. 4,498,010 to Biechler et al. which provides a virtual addressing technique and is incorporated by reference.
The above disclosures are directed to raster scan methods using a bit map of an image of the feature to be written, in which each pixel is typically either on or off. In contrast, in vector scan lithographic machines, shapes are defined in a stored image. It is well known in vector scan machines to alter the dosage (intensity) of a particular shape.
However the above referenced prior art does not provide, in the context of raster scan, a method for defining an edge of a feature to lie between pixel centers, i.e. to move the edge of the feature off the pixel grid, except by (1) use of multi-scan passes or (2) using virtual addressing to turn complete pixels on or off, producing unwanted edge roughness and pattern distortion. Each such pass requires a significant amount of time and hence reduces throughput of a raster scan machine, thus increasing the cost of the masks or integrated circuits formed thereby.
Warkentin et al. discloses in FIG. 7b (see col. 5, beginning at line 49):
By reducing the dosage of the light source as shown by graph 708, only desired area 705 is exposed to the exposure threshold level 703(b).
FIG. 7b shows the edge of a feature 702(b) in which the dosage of the laser beam source is reduced (modulated) for area 705 of the image. Warkentin et al. changes dose by reducing laser intensity.
Moreover, Warkentin et al. does not indicate that such an approach is suitable, and instead teaches the approach as shown in their FIG. 7c in which the level dose is reduced for all pixels exposed in one pass which are therefore of uniform intensity. Multiple passes of the beams, i.e. multiple scans, are used (see col. 5, lines 54 through 58.) Thus, Warkentin et al. teach a multi-pass system with the attendant reduction in throughput.